This invention relates to an apparatus for driving a motor for quickly and accurately locating (moving) a carrier or stage which is used, for example, in an apparatus for electron beam lithography and on which a sample (mask or wafer) is carried to a predetermined position.
In order to move the carrier or stage in a short time, high-speed pulses may be fed to a pulse motor driver which controls the motor coupled to the stage. However, when high-speed pulses at or above the self-start frequency of the pulse motor are suddenly applied to the driver, the motor operation becomes disordered to cause run-out or is subject to a start-stop (stuttering) operation with error or becomes out of control and does not rotate in spite of the injection of pulse current thereto in relation to the load torque. It is therefore common practice to use a so-called slow-up and slow-down system of starting and stopping in which the speed is gradually raised and lowered at start and prior to stop, respectively.
Heretofore, speed control has been effected in accordance with a selected linear function for the slow-up or slow-down operation. It has been empirically known, however, that the linear function cannot have an optimum shape for a moving mechanism, including the motor. Moreover, even when a linear function is utilized, there are limits imposed by the starting characteristics of the pulse motor itself, the magnitude of the load torque, etc., and a gradient at or above the speed at which run-out is incurred cannot be set. The prior art has been subject to such disadvantages, without being successful in rendering the speed of the carrier or stage movement higher.